The invention relates to a power steering apparatus, in particular, to a reaction pressure controller which controls an oil pressure acting upon a hydraulic reaction mechanism thereof.
In a power steering apparatus which is responsive to vehicle speed, as disclosed in Japanese Laid-Open Patent Application No. 103,765/1988, a control valve is operated in accordance with an input applied to a steering wheel, to introduce a pressure oil into one of chambers in a power cylinder in order to apply a steering assisting force to steerable road wheels. In order to allow a driver of the vehicle to sense the resistance to a steering operation, there are provided a hydraulic reaction mechanism in which a steering reaction is generated by causing a pressure oil to act upon the hydraulic reaction chamber and a reaction pressure control valve mechanism which controls the pressure introduced into the hydraulic reaction chamber.
A reaction pressure control valve mechanism is known in the art which controls a reaction pressure by means of a solenoid operated valve. Information such as a vehicle speed, a steering angle or the number of revolutions of an engine are input to a controller in order to control a displacement of the solenoid operated valve. In this instance, there must be provided some sort of sensor in order to derive such information, causing an increase in the manufacturing cost. In another variety, the solenoid valve is controlled in accordance with a vehicle speed alone to simplify the construction.
In an arrangement as described above in which the solenoid operated valve is controlled in accordance with the vehicle speed alone, there will be achieved a response which provideds a sharp steering sensation in a high load range (or a range of larger steering angles) covering from a medium to a high speed where an input torque is high, since then the relationship between the input torque and the cylinder pressure of the power steering apparatus will be relatively close to a linear one (see FIG. 5a). On the other hand, where a rapid steering operation is required, there is some times the likelihood that the steering operation may be retarded because of an increased loading upon the steering wheel. Hence, there is a drawback that such approach is not adapted to be incorporated into automobiles of advanced classes. This is due to the fact that the opening of the solenoid operated valve which is determined by the vehicle speed does not change for an equal vehicle speed regardless of loading upon the power steering apparatus, so that the reaction pressure increases with an increase in the cylinder pressure of the power steering apparatus (see FIG. 5b).